One of major obstacles that prevent an artificial light source from achieving high luminous efficacy is that the light source cannot convert energy into visible radiation without radiating a lot of infrared rays, of which the wavelengths are too long to sense with human eyes, at the expense of the visible radiation.
An incandescent lamp needs no ballasts, has a small size and a light weight, and shows a higher color rendering index than any other artificial light source. Due to these advantageous features, the incandescent lamp is a light source that is used most broadly worldwide. To increase the radiation efficiency of incandescent lamps, people tried to raise the operating temperature of the radiator or to find a radiator that has a small radiation in the infrared range. History teaches us that a carbon filament as a radiator material for an incandescent lamp was replaced by the currently used tungsten filament as a result of those efforts. By using the radiator of tungsten, the radiator could operate at a higher temperature than the radiator of any other material and therefore could reduce the percentage of radiations in the infrared range.
However, in spite of their efforts, the radiation produced by current incandescent lamps, using the tungsten filament, in the visible wavelength range is just 10% of the overall radiations thereof. The majority of the other radiations are infrared radiations, which account for as much as 70% of the overall radiations. Also, the current incandescent lamps cause a heat conduction due to an enclosed gap or a heat loss of 20% due to convection and have a luminous efficacy of about 15 lm/W, which is among the lowest ones in various artificial light sources. This performance of the incandescent lamps has not been improved significantly since 1930's.
Meanwhile, Patent Document No. 1 and other documents disclose a technique of drastically reducing the infrared radiations produced by a radiator and increasing the luminous efficacy of the lamp significantly. According to this technique, an array of very small cavities functioning as waveguides (which are termed “micro-cavities”) is provided on the surface of the radiator, thereby cutting down radiations of which the wavelengths exceed a predetermined value (e.g., infrared radiations) and selectively emitting only electromagnetic radiations with the predetermined wavelength. This patent document describes that cavities with a width of about 350 nm and a depth of about 7 μm are arranged at an interval of about 150 nm, thereby cutting down infrared radiations of which the wavelengths exceed about 700 nm. This patent document also describes that the luminous efficacy increases as much as six-fold at an operating temperature of 2,000 K to 2,100 K.
On the other hand, Patent Document No. 2 discloses a technique of cutting down the infrared radiations by winding a single metal fine wire around the filament.
Patent Document No. 1: Japanese Patent Application Laid-Open Publication No. 03-102701
Patent Document No. 2: Japanese Patent Application Laid-Open Publication No. 04-349338